Substituted 4,5-dihydro-6-(substituted)phenyl-3(2H)-pyridazinones and 6-(substituted)phenyl-3(2H)-pyridazinones useful as cardiotonic agents are the subject of U.S. Pat. No. 4,353,905 and U.S. application Ser. No. 477,695 filed Mar. 22, 1983 now 4,734,415. Additional references are cited therein to other known compounds.